The present invention relates to processing for representing amplitude of an output signal by one bit when a digitally processed signal of signal amplitude exceeding one bit as a result of predetermined signal processing on a digital signal represented by one bit is inputted and predetermined delta sigma modulation processing is performed on the digitally processed signal inputted.
A data format used for conventional digital audio has a sampling frequency of 44.1 kHz and a data word length of 16 bits, for example. In contrast, a high-speed one-bit audio signal resulting from delta sigma modulation has a very high sampling frequency of 64 times 44.1 kHz and a short data word length of one bit, for example. The high-speed one-bit audio signal resulting from delta sigma modulation thus features a broad transmittable frequency band as compared with the data format used for conventional digital audio. In addition, even with the one-bit signal, the delta sigma modulation enables a high dynamic range to be secured in an audio band of low frequencies as compared with the oversampling frequency of 64 times the sampling frequency. These features can be utilized to apply the high-speed one-bit audio signal to recorders and data transmission maintaining high sound quality.
The delta sigma modulation itself is not a particularly new technique. A delta sigma modulating circuit has conventionally been used often within an AD converter or the like because circuit configuration of the delta sigma modulating circuit is suitable for IC integration and accuracy of AD conversion can be secured relatively easily.
By passing the delta sigma modulated signal through a simple analog low-pass filter, an analog audio signal can be extracted.
In mixing delta sigma modulated one-bit signals of a plurality of channels, results of calculation of the plurality of channels obtained by multiplying the respective one-bit signals by respective predetermined mixing ratios are added together and then subjected to delta sigma modulation again to thereby generate a new one-bit signal. At this time, when the mixed signals are signals of highly correlated channels, an audio band component signal resulting from addition of the results of calculation of the plurality of channels obtained by multiplying the respective one-bit signals by the respective predetermined mixing ratios is increased in level. A delta sigma modulator capable of modulating a high signal level is therefore required.
However, in order to suppress an increase in quantization noise level due to remodulation, a delta sigma modulator of high order such for example as fifth order is used as a delta sigma modulator for performing the remodulation. In this case, a maximum percentage modulation of about 50% is normally used. For system stability, a high percentage modulation exceeding 50%, for example, cannot be obtained in a high-order delta sigma modulator.
On the other hand, a first-order delta sigma modulator can perform modulation up to 100% modulation. However, the first-order delta sigma modulator cannot be used because quantization noise level in an audio band is increased.
Thus, in order to mix signals without limit processing, there is no choice but to lower the signal level of each channel and limit the percentage modulation at the time of the delta sigma remodulation. As a result, the quantization noise level is increased relative to the lowered signal level, thus lowering S/N ratio of an audible band.
In addition, in a digital signal processing device formed with requantizing means for requantizing multi-bit data outputted by quantizing means, the provision of each of the quantizing means increases circuit scale.